What methods do you know to set the EIGRP metrics when redistributing?

You can set the default metric for all redistribute command:
(router EIGRP)default-metric

Or set the metric for a single redistribute command :(router EIGRP) redistribute ospf 1 metric

Or you can set different metrics for each routes which are redistributed in a single redistribution by route-map: redistribute ospf 1 route-mapmetric This can override the previous 2 options.

All redistributed routes will have the same feasible distance since they use the same component metric (configured above).

What additional meaning does this command have: redistribute ospf 2 ?

It redistributes routes learned from OSPF 2 and also learns the connected routes for interfaces on which OSPF 2 process had been enabled. (It will be an external route on the other EIGRP devices).

What is the EIGRP AD (internal and external)? When can be external EIGRP AD seen in routing table?

EIGRP internal AD: 90 external: 170

External AD can be seen for example when something is redistributed into EIGRP.

Redistribution into OSPF

What are the default metrics when redistributing into OSPF (BGP, RIP/EIGRP/IS-IS, OSPF)?

Redistributing BGP: metric = 1

Redistributing other OSPF process: metric = source metric

Redistributing from all other source: metric = 20

What problem can cause OSPF when using Loopbacks for simulating networks?

For example OSPF realises that the /24 Loopback is still a loopback and it’s really /32 because it isn’t a whole network.So when it is redistributed it seems like this on the other router:To resolve this issue we can use this command in the Loopbacks configuration section:

int lo 3
ip ospf network point-to-point

After this we will see as a /24 network on the other router (where it is redistributed):

How do you configure metric when redistributing to OSPF?

(ospf)default-metric cost

(ospf) redistribute protocolnumber metric cost

use route-map on the redistribute command

What is the difference between E1 and E2 metric types?

for E1 both external and internal cost matter to the choice of best route

E1 is better if you have multiple exit points in the area.

for E2 only the external cost matter (except a tie need to be broken)

NOTE: When 2 E2 routes have the same metric (usually 20 when redistributing), OSPF will use the Forwarding Metric. This way E2 will act as an E1 route.

What is Type 4 LSA?

it lists RID of ASBR and RID of ABR

Type 4 LSA lists the ABR’s cost to reach the ASBR

If a R1 knows 2 routes to the same E2 destination (with same metric) and the ASBR is within the area, how will R1 choose?

It will find the ASBR listed in the Type 5 LSAs

Calculates the lowest-cost route to reach any of the ASBRs based on the intra-area LSDB topology.

use the next hop based on the best route to reach ASBR

If a R1 knows 2 routes to the same E2 destination (with same metric), but the ASBR is in another area, how will R1 choose?

it will find the ASBR listed in the Type 5 LSA

calculates the lowest-cost route to reach any of the ASBRs

to do this it will use Type 3 LSA because the ASBR is in another area

it finds the ABR with the best metric and use that route

the router will add the metric of E2, ASBR and ABR and choose the lowest

What is the default redistribute metric type and how can you set the other one?

default = E2

(ospf) redistribute eigp 1 subnets metric-type 1

What if the ASBR is in another area?

the router will calculate as in the case of E2

adds the best intra-area cost to reach ABR

adds the cost from ABR to ASBR (info in LSA Type 4)

adds the external cost (LSA Type 5)

What happens if you redistribute into a NSSA area?

there will be Type 7 LSAs

if these Tpye 7 LSAs are propagated into another area they will change to Type 5 LSA

Redistribution with Route Maps

What is a Rotue Map able to?

identifies routes and able to change them

makes filtering choices which routes are redistributed and which are not

set the metric

set the type of external route (OSPF ie)

set a route tag (unitless integer) which can be later used

How can you filter the redistributed routes with the redistribute command?

What does the permit and deny mean in the route-map? How does it work?

route-map TEST permit 10
Each sequence of a route map can be permited or denied. The permit means that the route (selected by the match clause) will be redistributed (additionally many values can be set in the SET section of the sequence). The denied routes will be filtered.

What can be set by route-map?

The route metric: set metric

An EIGRP route metric (more values): set metric

The type of route (OSPF): set metric-type { type-1 | type-2}

A uniteless tag: set tag tag-value

Does route-map has implicit deny? How do ACLs work with route-maps? What should we not use in ACLs?

ACL is used for selecting a specific route. So denying anything in ACL has no point. Routes go through on route-map one by one and they can match with routes selected by ACL ( if ACL denies something that is like an empty line for route-map, useless ).

There is no implicit deny in route-maps but if a route has no match then it won’t be redistributed.

When we using route-maps to set something on some routes then it is important to use a last sequence which permits all other non-matched routes to be redistributed:route-map TEST permit 100
(match part is empty -> matches everything which haven’t been matched yet)

Which route will the router choose if it has multiple choices (out of domain routes)?

The one with the lower AD. If the routes have same AD then it will use the metric as tie breaker.

What will defeat the domain loop when redistributing between OSPF and EIGRP?

The AD: EIGRP internal 90 < OSPF 110 < EIGRP external 170

What will defeat the domain loop when redistributing between EIGRP and RIP?

The AD: EIGRP internal 90 < RIP 120 < EIGRP external 170

What will defeat the domain loop when redistributing between RIP and OSPF?

RIP can be set with only one AD but OSPF understands external routes, intra-area routes and inter-area routes.

How do you change the AD value of RIP, EIGRP but especially of OSPF?

RIP: distance

EIGRP: distance eigrp { internal-ad | external-ad }

OSPF: distance ospf { external-ad | intra-area-ad | inter-area-ad }

When start the AD become problematic?

With redistribution between more than 2 routing protocols.

How can you solve the problem on the previous topology?

There is an option to set an individual AD for a given router (which advertises routing information ie:RD2):distance
This can be extended with an ACL. This way only given routes AD will be changed even from the specific router.

Another way is to filter the redistributed routes with a route-map, so what’s already redistributed from an area wont be redistributed back:

The routes can be tagged by a route-map and later the tagged route can be matched with another route-map. This way we can follow a specific route advertisement.

What does Prefix List do and why do we need it?

Prefix list filter routes like Distribute List but it is used in a much bigger scale.
Mostly we use it with BGP. For example when there are 2 huge ISP and they exchange routes they have to summarize the routes or else the routing table would overflow. To be sure that the other ISP don’t advertises too specific routes we can use Prefix Lists:ip prefix-list TEST permit 23.40.0.0/16 le 20
The previous prefix lists mean: we allow routes with the 23.40.0.0 /16 subnet but maximum with /20 prefix. 23.40.10.0/24 or 23.40.10.10 is not allowed but 23.40.1.0/19 is allowed.